Method and apparatus for temporal and spatial beam integration

ABSTRACT

A method and apparatus for providing sequential temporal and spatial integration of a collimated non-symmetrical excimer laser beam to optimize the temporal and spatial characteristics of the beam. The temporal integrator comprises a pair of cylindrical lenses spaced along the beam axis by a distance substantially equal to the sum of the focal length of both lenses, and a motor mechanism for rotating the two spaced cylindrical lenses about the beam axis. The spatial beam integrator includes a plurality of prisms distributed about a hollow center, the outlet face of each prism being angled with respect to the body axis of the spatial beam integrator so that portions of the laser beam passing through a given prism are refracted towards the center upon emergence from the outlet face. The spatial beam integrator is preferably rotated about the beam axis at twice the rotation rate of the cylindrical lenses so that the rotated beam emerging from the temporal beam integrator is stationary with respect to the spatial beam integrator. Alternatively, the spatial beam integrator may be rotated at the same rate as the cylindrical lenses, or may be maintained stationary, i.e., not rotated at all.

This application is a division of application Ser. No. 08/368,799 filedJan. 4, 1995 now U.S. Pat. No. 5,646,791.

BACKGROUND OF THE INVENTION

This invention relates to optical beam delivery systems in general, andto optical beam delivery systems used with laser beams to optimize thetemporal and spatial characteristics thereof.

Optical beam delivery systems are known which are used to improve thetemporal and spatial characteristics of collimated beams of radiationwith non-symmetrical energy profile cross sections, such as excimerlaser beams. For example, in the Visx Twenty/Twenty Excimer Laser Systemdeveloped by Visx Incorporated of Santa Clara, Calif., a collimatedlaser beam used for photorefractive keratectomy (PRK) andphototherapeutic keratectomy (PTK) is delivered to the plane of surgeryby means of an optical beam delivery system which provides both spatialand temporal integration for an excimer laser beam. In this system, acollimated laser beam is first passed through a stationary spatial beamintegrator comprising a plurality of prisms, which are preferablyhexagonal in shape, distributed about an optical center in the form of asimilar hollow space, one face of each prism being angled with respectto the central axis so that portions of a laser beam passing througheach prism are refracted toward the central axis of the prism assembly.After passing through the spatial beam integrator, the laser beam isnext transmitted through a temporal beam integrator comprising a doveprism which is rotated about the longitudinal optical axis in order torotate the beam. The beam emerging from the temporal beam integrator isthen directed through a variable diameter aperture and delivered to thesurgical plane by means of appropriate mirrors and lenses.

While highly effective in providing spatial and temporal integration toa collimated laser beam, this arrangement is extremely sensitive to theplacement of the dove prism along the optical axis of the beam deliverysystem. In particular, any slight misalignment of the dove prism resultsin a multiplication of the angular error by a factor of 2. Since anyangular deviations radially displace the overlapping beam relative tothe aperture, thereby affecting symmetry of the beam at the treatmentsite, extreme care must be taken in initially aligning the dove prismwith respect to the beam axis and frequent periodic alignment checksmust be made to ensure that the initial alignment has not beendisturbed. Efforts to provide a spatial and temporal beam integrationtechnique devoid of this disadvantage have not met with success to date.

SUMMARY OF THE INVENTION

The invention comprises a technique for temporally and spatiallyintegrating a collimated laser beam which is relatively easy toinitially align with respect to the beam axis, and which is relativelyinert and insensitive to angular misalignment of the optical elementswhich perform the temporal beam integration.

From a process standpoint, the invention comprises a method ofprocessing a collimated laser beam to improve the spatial and temporalcharacteristics thereof, the method including the steps of first passingthe collimated beam through a temporal beam integrator to rotate thebeam about the axis thereof at a predetermined rate, and then passingthe rotating beam emerging from the temporal beam integrator through aspatial beam integrator to effect spatial integration thereof. The stepof passing the collimated beam through a temporal beam integratorpreferably includes the steps of positioning a pair of cylindricallenses arranged in spaced relationship along the axis of the collimatedbeam, and rotating the pair of cylindrical lenses in unison about thebeam axis. The effect of this temporal integrator mechanism is arotation of the laser beam at a rotational speed of twice the speed ofrotation of the cylinder lens pair. In the preferred embodiment, thecylindrical lenses are substantially identical. In one embodiment, thespatial beam integrator is rotated about the beam axis at an angularspeed greater than the speed of rotation of the cylindrical lenses,preferably at a speed which is twice the rate of rotation of thecylindrical lenses so that the follow-on spatial beam integrator isrelatively stationary with respect to the rotating beam emerging fromthe cylindrical lenses. In another embodiment, the angular speed ofrotation of the spatial beam integrator is made equal to the speed ofrotation of the cylindrical lenses. In still another embodiment, thespatial beam integrator is maintained stationary, i.e., not rotated atall.

The first embodiment of the method preferably includes the initial stepof rotating the spatial beam integrator about the beam axis beforecommencing rotation of the temporal beam integrator in order toinitially optimize the spatial characteristics of the collimated beamtransmitted through the spatial beam integrator.

From an apparatus standpoint, the invention comprises a laser beamdelivery apparatus for temporally and spatially integrating a collimatedlaser beam, the beam delivery apparatus including a pair of cylindricallenses arranged in spaced relationship along the axis of the collimatedlaser beam, with the cylinder axes of the cylindrical lenses beingsubstantially aligned. The cylindrical lenses are preferably spacedalong the beam axes by an amount substantially equal to the sum of thefocal distance of each cylindrical lens. A spatial beam integrator ispositioned in the path of the beam emerging from the cylindrical lenses.

In the preferred embodiment, two cylindrical lenses of equal refractivepower, with their axes aligned and their separation equal to the sum oftheir focal distances, are installed in the path of the laser beam. Thisarrangement provide a substantially equally formed, but rotating, laserbeam at the exit of the integrator.

In other embodiments, two cylindrical lenses of unequal refractivepower, with their axes aligned and their separation equal to the sum oftheir focal distances, are installed into the laser beam. Thisarrangement provides an increased or reduced, but equally rotating,laser beam at the exit of the integrator. The size of the laser beamexiting from this integrator will be affected in width and height by theinverse of the ratio of the first and second integrator lenses and thesine or cosine function of the angle of the first lens to the angle ofthe laser beam entering such integrator.

The temporal integrator apparatus includes first means for rotating thecylindrical lenses about the beam axes in unison so that a beam passingthrough the pair of cylindrical lenses is rotated about the beam axis attwice the rotational speed of the lenses. In the preferred embodiment ofthe invention, the apparatus includes means for providing relativerotation between the spatial beam integrator and the pair of cylindricallenses. The providing means preferably includes second means forrotating the spatial beam integrator relative to the cylindrical lenses,and means for providing synchronous motion between the first and secondrotating means. The angular speed of the spatial beam integrator ispreferably set to be a multiple, preferably 2, of the angular speed ofrotation of the cylindrical lenses.

The spatial beam integrator preferably comprises a plurality ofhexagonal prisms distributed about a center, with each prism having alight outlet face for refracting an emerging portion of the collimatedbeam towards the center of the prism assembly, each light outlet facebeing preferably positioned at an angle with respect to a body axispassing through the center of the spatial beam integrator. The centermay comprise either a hollow space or an optical element such as a prismhaving a flat light outlet face.

The apparatus further preferably includes means for permitting initialrelative rotation between the spatial beam integrator and thecylindrical lenses in order to optimize the spatial characteristics ofthe collimated beam passing therethrough. The invention further mayinclude an expanding lens, preferably a spherical lens, positioned inthe path of the beam emerging from the downstream one of the pair ofcylindrical lenses, preferably between that lens and the spatial beamintegrator.

The first means for rotating the cylindrical lenses about the beam axispreferably includes a housing for mounting the cylindrical lenses inproper alignment, a motor for generating mechanical motion, and meansfor transferring the mechanical motion to the housing. The transferringmeans preferably comprises a driving gear coupled to the motor and adriven gear coupled to the housing and engagable with the driving gear.The means for providing relative rotation between the spatial beamintegrator and the cylindrical lenses preferably comprises a secondhousing for mounting the spatial beam integrator, a motor for generatingmechanical motion, and means for transferring the mechanical motion tothe second housing, the transferring means preferably comprising adriving gear coupled to the motor and a driven gear coupled to thehousing and engagable with the driving gear. The motor is preferably asingle motor shared between the first rotating means and the providingmeans.

In an alternate embodiment of the invention, the spatial beam integratoris rotated at the same rate as the cylindrical lenses. In anotheralternate embodiment, the spatial beam integrator is fixed and thecylindrical lenses are rotated. In both of the alternate embodiments,the angular position of the rotated beam with respect to the spatialbeam integrator varies with respect to time; while in the preferredembodiment, the angular position of the rotated beam is fixed withrespect to the spatial beam integrator.

The invention provides both spatial and temporal integration for acollimated laser beam and is substantially less sensitive tomisalignment of the temporal beam integrator with respect to the beamaxis. In particular, any off axis misalignment results in multiplicationby a factor of approximately 0.5 times the offset, due to the use of therefraction principle of the cylindrical lenses, which compares favorablyto the multiplication factor of 2 encountered with temporal beamintegrators employing dove prisms.

For a fuller understanding of the nature and advantages of theinvention, reference should be had to the ensuing detailed descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a portion of a laser beam opticaldelivery system incorporating the invention;

FIG. 2 is a schematic sectional view taken along lines 2--2 of FIG. 1 ofa portion of the spatial beam integrator;

FIG. 3 is a sectional view of a preferred embodiment of the inventiontaken along lines 3--3 of FIG. 4;

FIG. 4 is an end view of the preferred embodiment of the invention;

FIGS. 5A and 5B together constitute a schematic diagram of a laser beamoptical delivery system incorporating the invention; and

FIG. 5C illustrates the relative orientation of FIGS. 5A and 5B.

FIG. 6 is a schematic diagram of a temporal beam integrator using twocylindrical lenses of different focal length.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to the drawings, FIG. 1 illustrates in schematic form alaser beam delivery apparatus according to the invention. As seen inthis figure, a collimated beam 10 from a laser source (not shown) isdirected onto the inlet face of a temporal beam integrator generallydesignated with reference numeral 12. In the preferred embodiment ofFIG. 1, temporal beam integrator 12 includes a pair of substantiallyidentical cylindrical lenses 13, 14 each arranged in the path of beam 10and spaced along the beam axis by a distance equal to the sum of thefocal distances of the lenses. The cylindrical axes 15 of each of thelenses 13, 14 are aligned with respect to each other, and each lens isarranged with the flat face normal to the beam axis, with the opticalcenter of each lens 13, 14 coincident with the beam axis. The convexcylindrical surface of lens 13 provides the inlet face for temporal beamintegrator 12, while the convex face of cylindrical lens 14 forms theoutlet face of the temporal beam integrator.

As suggested by broken line 17, cylindrical lenses 13, 14 aremechanically linked, and as suggested by circular arrow 18, cylindricallenses 13 and 14 are mounted for synchronous rotation about the beamaxis. When a beam 10 passes through temporal beam integrator 12 as thelenses 13, 14 are rotated in unison, the rotated beam emerging from theoutlet face of lens 14 is rotated twice for each complete revolution ofthe lens pair 13, 14.

An optional beam expanding lens 20 is positioned in the path of therotated beam emerging from the temporal beam integrator 12 and is usedto expand the beam size in those applications requiring such beamexpansion.

A spatial beam integrator generally designated with reference numeral 25is located in the path of the rotating beam emerging from temporal beamintegrator 12 (and optionally emerging from the optional beam expanderlens 20). Spatial beam integrator 25 comprises a close packed array ofhexagonal prisms 27 clustered about the center of spatial beamintegrator 25. As shown in FIG. 2, the outlet face 28 of each of theprisms 27 is angled with respect to the central axis 29 of the spatialbeam integrator. As a consequence, that portion of the rotated laserbeam passing through each prism is refracted towards the central axisupon emergence from the outlet face 28. The spatially integrated beamemerging from spatial beam integrator 25 is transmitted to follow onoptical elements and to the destination site or plane.

As suggested by curved arrow 32, spatial beam integrator 25 may bemounted for rotational movement about the beam axis. In the preferredembodiment, spatial beam integrator 25 is mounted for rotation in thesame angular direction as temporal beam integrator 12, but at twice therotational rate of the temporal beam integrator 12. Thus, the rotatedbeam emerging from the temporal beam integrator 12 has a fixed angularorientation with respect to spatial beam integrator 25 (since the beamis rotated by a factor of 2 in passing through the two cylindricallenses 13, 14). In this embodiment, the angular orientation of spatialbeam integrator 25 is initially adjusted with respect to the angularorientation of temporal beam integrator 12 with integrator 12 stationaryin order to determine the angular position of spatial beam integrator 25relative to beam 10 which affords the optimum spatial characteristics,i.e., smoothness, profile and homogeneity. Once this orientation hasbeen determined, the relative angular positions of temporal beamintegrator 12 and spatial beam integrator 25 are controlled duringrotation of these two units such that this optimum angular orientationbetween the beam 10 and the spatial beam integrator is maintainedconstant. In this way, the spatial beam integration is optimized.

In a first alternate embodiment of the invention, spatial beamintegrator 25 is simply locked to temporal beam integrator 12 androtated in unison therewith. In still another alternate embodiment, theangular position of the spatial beam integrator 25 is simply fixed andonly the temporal beam integrator 12 is rotated. In both of thesealternate embodiments, the rotated beam emerging from temporal beamintegrator 12 also rotates with respect to spatial beam integrator 25.As a consequence, the initial angular alignment of spatial beamintegrator 25 with respect to temporal beam integrator 12 isunnecessary.

FIGS. 3 and 4 illustrate a preferred embodiment of the apparatus formounting cylindrical lenses 13, 14 and spatial beam integrator prisms27, and for rotating prisms 27 relative to lenses 13, 14. As seen inthese figures, cylindrical lens 13 is mounted in an aperture 41 of ahollow, generally cylindrical member 42. Cylindrical lens 14 is mountedin an aperture 44 in a second generally cylindrical member 45. Member 42has an outer diameter sized to provide a translatable sliding fit withinthe inner diameter of member 45 so that the axial separation distancebetween lenses 13 and 14 may be adjusted.

Mounting member 45 is rotatably mounted by means of bearings 46 to asupport member 48. Support member 48 also carries a drive motor 50, amotor transmission mechanism 51 and an output shaft 53. A first drivinggear 55 is mounted on shaft 53 and held in place by a friction clamp 57which is received about a friction flange 59 attached to one face ofdriving gear 55. A second driving gear 61 is also mounted on shaft 53 bymeans of a friction clamp 57 and flange 59.

Driving gear 55 is enmeshed with a first driven gear 64 which is securedto housing member 45. Driving gear 61 is engaged with a second drivengear 66 which is secured to a mounting head 69 for spatial beamintegrator prisms 27.

In use, cylindrical lenses 13, 14 are arranged within their respectiveapertures in members 42, 45 with their cylindrical axes aligned, and theseparation distance along the beam axis is adjusted until lenses 13, 14are separated by a distance equal to the sum of the focal distances ofboth lenses. Next, the array of hexagonal prisms 27 is mounted in member69, and this assembly is attached to driven gear 66. This assembly isnow aligned with the axis of the laser beam (indicated by the phantomline in FIG. 3), after which the laser beam profile is examined whilerotating mounting heady 69. Once the optimum relative angular positionbetween the beam and the prisms 27 is attained driving gear 61 is lockedto shaft 53 by means of clamp 57 and 59, and driving gear 55 is likewiselocked to shaft 53 (unless this step was already done prior to theinitial rotational adjustment of mounting head 69). The apparatus is nowaligned and ready for use.

In use, motor 50 is operated by appropriate control signals to rotatedriving gears 55, 61, and thus rotate housing members 42, 45 in bearings46 and prisms 27. The relative rates of rotation of the lenses 13, 14with respect to the prisms 27 are governed by the gear ratios of gears55, 61, 64 and 66. As will be appreciated by those skilled in the art,these relative rates of rotation can be changed by simply using gearswith different ratios, as dictated by the requirements of any particularapplication.

FIGS. 5A and 5B illustrate the application of the invention to anophthalmological laser surgery system. FIG. 5C illustrates the relativeorientation for FIGS. 5A and 5B. As seen in these figures, a collimatedbeam 10 from a suitable laser source 70, such as an excimer laser beamsource for generating a laser beam in the far ultraviolet range with awavelength of 193 nanometers, is directed to a beam splitter 71. Part ofthe beam is reflected onto an energy detector 72; the remaining portionis transmitted through the beam splitter 71 and reflected by a mirror 73onto the inlet cylindrical face of the temporal beam integrator 12. Therotated beam emerging from integrator 12 is passed through expandinglens 20, which is a negative lens for slightly expanding the beam size,thence through spatial beam integrator 25 and onto a mirror 74. The beamreflected by mirror 74 is passed through a collimating lens 75,preferably a plano convex positive lens which reduces the beam size. Thebeam emanating from collimating lens 75 is directed onto a variableaperture 77, which is preferably a variable diameter iris combined witha variable width slit used to tailor the beam size and profile to aparticular ophthalmological surgery procedure, such as a photorefractivekeratectomy procedure. The apertured beam from variable aperture 77 isdirected onto an imaging lens, preferably a biconvex singlet lens with afocal length of 125 mm. The imaged beam from lens 79 is reflected by amirror/beam splitter 80 onto the surgical plane 82 at which the apex ofthe cornea of the patient is positioned. A treatment energy detector 84senses the transmitted portion of the beam energy at mirror/beamsplitter 80. Beam splitter 86 and a microscope objective lens 88 arepart of the observation optics. If desired, a video camera may beinstalled in the optical path of the apertured beam emanating from themicroscope objective lens 88 to assist in viewing or recording thesurgical procedure. Similarly, a heads-up display may also be insertedin the optical path of the microscope, reflecting from the beam splitter86 to provide an additional observational capability.

In the application of the invention shown in FIGS. 5A-C, the speed ofrotation of the temporal beam integrator is generally dependent upon thenature of the surgical procedure, and is specifically related to therate at which the laser pulses are generated. In general the rotationrate ranges from about 100 to about 200 revolutions per minute inophthalmological surgical procedures.

As noted above, cylindrical lenses 13, 14 of temporal beam integrator 12in the preferred embodiment described above are substantially identicaland thus have equal focal lengths. If desired, cylindrical lenses havingdifferent focal lengths may be employed as shown in FIG. 6. Withreference to this figure, two cylindrical lenses 113, 114 of unequalrefractive power are arranged with their axes aligned as shown. Lenses113, 114 are spaced along the beam axis by a distance equal to the sumof the two focal distances fx, fy. In this embodiment, the size of thelaser beam exiting from the exit side of the temporal beam integratorwill be affected in width and height by the inverse of the ratio of thefirst and second integrator lenses 113, 114, and the sine or cosinefunction of the angle between the entering laser beam and the entrancelens. As will be understood by those skilled in the art, in the FIG. 6embodiment, either lens 113 or 114 may serve as the entrance lens or theexit lens for the temporal beam integrator. Similarly, it is understoodthat lenses 113, 114 are arranged and operated in the same manner asthat described above with respect to the embodiments of FIGS. 1-5A-C.

The temporal and spatial beam integrator of the invention affords anumber of advantages over the known spatial and temporal beam integratoremploying the rotating dove prism. Firstly, due to the use ofsubstantially identical cylindrical lenses 13, 14, and the simplemounting arrangement illustrated in FIGS. 3 and 4, the temporal andspatial beam integrator optics can be relatively aligned initially.Further, once aligned, the probability of subsequent misalignment isextremely low. Also, any angular misalignment with respect to the laserbeam axis results in a multiplication of the misalignment error on thelaser beam by only a factor of approximately 0.5, which comparesfavorably to an error multiplication factor of 2.0 for a temporal beamintegrator using a rotating dove prism.

While the above provides a full and complete disclosure of the preferredembodiments of the invention, various modifications, alternateconstructions and equivalents will occur to those skilled in the art.For example, while the invention has been described with expressreference to an ophthalmological laser surgery system, otherapplications of the invention may be made, as desired. Therefore, theabove should not be construed as limiting the invention, which isdefined by the appended claims.

What is claimed is:
 1. A method of processing a collimated laser beam toimprove the spatial and temporal characteristics thereof, said methodcomprising the steps of:(a) first passing the collimated beam through atemporal beam integrator to rotate the beam about the axis thereof at apredetermined rate; and (b) then passing the rotating beam emerging fromthe temporal beam integrator through a spatial beam integrator to effectspatial integration thereof.
 2. The method of claim 1 wherein said step(a) includes the steps of positioning a pair of cylindrical lenses inspaced relationship along the axis of the collimated beam, and rotatingsaid pair of cylindrical lenses in unison about the beam axis.
 3. Themethod of claim 2 wherein said step of positioning includes the step ofspacing said pair of cylindrical lenses along the beam axis by an amountsubstantially equal to the sum of the focal distances of bothcylindrical lenses.
 4. The method of claim 3 wherein said cylindricallenses have substantially equal refractive power.
 5. The method of claim3 wherein said cylindrical lenses have different focal lengths.
 6. Themethod of claim 2 wherein said step (b) includes the step (i) ofrotating the spatial beam integrator about the beam axis.
 7. The methodof claim 6 wherein said step (i) includes the step of rotating thespatial beam integrator at an angular speed greater than the speed ofrotation of the cylindrical lenses.
 8. The method of claim 7 wherein theangular speed of the spatial beam integrator is twice the speed ofrotation of the cylindrical lenses.
 9. The method of claim 6 wherein theangular speed of rotation of the spatial beam integrator is equal to thespeed of rotation of the cylindrical lenses.
 10. The method of claim 1wherein said step (b) includes the step of maintaining the spatial beamintegrator stationary with respect to the rotation of the beam.
 11. Themethod of claim 1 wherein said step (a) is preceded by the step ofrotating the spatial beam integrator about the beam axis to initiallyoptimize the spatial characteristics of the collimated beam passingtherethrough.